The revenue generated by an aircraft is directly related to the number of passengers and the amount of cargo it can carry. The greater the passenger seating and cargo space, the greater the potential revenues. One method of increasing the passenger and cargo capacity of an existing aircraft design is to increase the length of its fuselage.
An aircraft fuselage has three main parts, a forward fuselage body, middle fuselage body and aft fuselage body each comprising one or more generally cylindrical shaped “sections”. A section having a constant cross-section is called a “constant section” and is defined as existing where the size and geometry of the fuselage perimeter is substantially unchanged between longitudinally adjacent fuselage frames or other longitudinally adjacent fuselage perimeter structure. Exemplary shapes of “constant section” include circular, piecewise circular, elliptical and non-circular cross-section, with a right circular cylinder being one specific example. A section having a tapering cross-section like a cone, ellipsoid, catenoid or a spline surface of revolution or Bezier surface of revolution, for example, is a “tapered section”. One or more constant sections are joined together with one or more tapered sections to form a complete fuselage.
The forward fuselage body or forward body comprises a tapered section containing a cockpit or flight deck and aircraft nose (“nose section”) and a forward constant section. The middle fuselage body or middle body comprises a middle constant section to which the wings are typically attached using a wing-to-body join. The middle constant section also commonly includes a main landing gear wheel well or housing, for aircraft having retractable landing gear. The aft fuselage body or aft body comprises an aft constant section and a tapered section having a tail assembly or empennage (“tail section”). The bottom portion or underside of the tail section is typically curved upward or “upswept”.
In order to create a longer or stretched aircraft, one or more constant sections, usually having a uniform length (“plug sections”), are typically inserted as plugs or length enhancements between the middle body and forward body and also between the middle body and aft body. The number of plugs used depends on the desired size and configuration of the stretched aircraft. Note that plugs are not necessarily structurally separate members, and may be structurally integral with either of the adjacent constant sections. One example of a stretched aircraft is the Boeing 737-900ER which is a stretched version of the older and shorter Boeing 737-700.
Stretching an aircraft reduces the tailstrike angle of the stretched aircraft. As used herein, the term “tailstrike” refers to an event in which the aft body of an aircraft, typically the tail section, strikes the runway during take-off or landing. The term “tailstrike angle” means the angle at which the aircraft's nose section is raised or pitched upward relative to a baseline angle, wherein a tailstrike (aft body or tailskid contact with the ground) will occur during takeoff and/or landing and with main landing gear oleos extended or compressed. The baseline angle can be horizontal or can be the fuselage reference angle with the aircraft resting on all its landing gear on the ground. A tailstrike can occur during takeoff if the pilot pulls up (“rotates”) too rapidly or at too sharp an angle or at too low an airspeed, leading to the aft body of the aircraft hitting the runway. A tailstrike occurs during landing if the pilot raises the nose of the aircraft (“flares”) too high when touching down on the runway, or lands at too low an airspeed. For example, the tailstrike angle of a Boeing 737-800 aircraft is just over 9 degrees with main gear oleo compressed and around 11 degrees with main gear oleo extended. An aircraft suffering a significant tailstrike may have to be inspected, repaired and certified flightworthy before it can return to service resulting in delay, repair costs and lost passenger and cargo revenue. Airlines can and do mitigate tailstrike risk on takeoff and landing by reducing the payload on particular aircraft missions, by artificially limiting the number of passengers or tonnage of revenue cargo—but this payload reduction causes a severe economic penalty.
The aerospace industry has several solutions for reducing the probability of the occurrence of a tailstrike or at least reducing the damage caused by a tailstrike. To prevent tailstrikes, stretched aircraft can be fitted with longer or semi-levered main landing gear or with tilting main landing gear bogies. However, this solution requires modifications to the landing gear or totally new landing gear. It may also require re-designing the wheel well to accept the modified or new landing gear. Physical tailskids can also be added to absorb shock and mitigate body damage in the event of a tailstrike. However, this adds undesirable weight to the aircraft. Also, this solution requires a re-design of the tail section. Aircraft can be fitted with an “electronic tailskid” or “supplemental electronic tailskid system” (“SETS”) that applies flight control command inputs to the elevators to avoid tailstrike or reduce tailstrike closure rate (the angular rate at which the aft body approaches the ground surface) when conditions corresponding to an incipient tailstrike are detected by sensors and/or detection algorithms. However, this solution requires fitting complex sensors and electronic systems to the aircraft. This solution cannot readily be used by stretched aircraft that do not have fly-by-wire flight control systems.
It is therefore desirable to provide additional methods for reducing the probability of an aircraft suffering a tailstrike, which do not require extensive re-design or modification of an aircraft, that are less costly, and that can be used in aircraft not having a fly-by-wire system. It is also desirable to enable the aircraft to carry a larger payload on critical missions, without needing to reduce payload to keep tailstrike risk at an acceptable level.